This organism consists of a single cell with several flagella. Is it a prokaryote, such as a bacterium? Actually, it’s larger than a prokaryotic cell, and it also has a nucleus. Therefore, this organism belongs to the domain Eukarya, the domain that includes humans. This particular eukaryote is one of the smallest, simplest organisms in the domain, called a protist. It’s scientific name is
Giardia lamblia.
As a human parasite, it can make us sick.

Kingdom Protista

Protists are a group of all the eukaryotes that are not fungi, animals, or plants. As a result, it is a very diverse group of organisms. The eukaryotes that make up this kingdom, Kingdom
Protista
, do not have much in common besides a relatively simple organization. Protists can look very different from each other. Some are tiny and unicellular, like an
amoeba
, and some are large and multicellular, like
seaweed
. However, multicellular protists do not have highly specialized tissues or organs. This simple cellular-level organization distinguishes protists from other eukaryotes, such as fungi, animals, and plants. There are thought to be between 60,000 and 200,000 protist species, and many have yet to be identified. Protists live in almost any environment that contains liquid water. Many protists, such as the
algae
, are photosynthetic and are vital primary producers in ecosystems. Other protists are responsible for a range of serious human diseases, such as
malaria
and sleeping sickness.

The term protista was first used by Ernst Haeckel in 1866. Protists were traditionally placed into one of several groups based on similarities to a plant, animal, or fungus: the animal-like
protozoa
, the plant-like
protophyta
(mostly algae), and the fungus-like
slime molds
and
water molds
. These traditional subdivisions, which were largely based on non-scientific characteristics, have been replaced by classifications based on
phylogenetics
(evolutionary relatedness among organisms). However, the older terms are still used as informal names to describe the general characteristics of various protists.

Protists range from single-celled amoebas to multicellular seaweed. Protists may be similar to animals, plants, or fungi.

Summary

Kingdom Protista includes all eukaryotes that are not animals, plants, or fungi.

Kingdom Protista is very diverse. It consists of both single-celled and multicellular organisms.